This invention relates to an electromagnetic interference prevention cable. More specifically, a high-frequency interference prevention and/or electromagnetic wave induction prevention wire is used for electrical connection of an electronic device such as an audio device and an office automatic device.
In conventional electromagnetic and high-frequency circuits, various kinds of shield cables and shield plates have been used in order to prevent a wrong operation due to noises produced from such circuit.
In the conventional high-frequency interference prevention, a static coupling and an electromagnetic coupling between the wires is interrupted by a shield cable or a shield plate, thereby removing unnecessary oscillation.
However, such method requires a highly-technical layout of shield cables and shield plates, and can not actually be achieved easily.
In recent years, a computer control for electric devices and electric products has been remarkable. Electronic circuits of such devices have been highly integrated, and current flowing through elements have been microscopic, and there has arisen a problem that a wrong operation of the device may occur due to induction between wires of a wiring bundle.
On the other hand, the products have become compact and lightweight, and also the space-saving and lightweight design of the wiring has been strongly desired.
There is known a shield cable of this kind as shown in FIG. 11 in which an insulation layer 102, a shield layer 104 and a covering insulation layer 105 are provided around an outer periphery of a central conductor 101, and a drain wire 103 is provided along the shield layer 104 so as to facilitate an earth connection operation (Japanese Utility Model Application Examined Publication No. Sho. 53-48998). The shield layer 104 is made of electrically conductive metal such as a metal braid and a metal foil.
In the conventional shield cable with the drain wire, a wire (conductor) of a circular cross-section is used as the drain wire 103, and therefore the diameter of the shield cable becomes large. This has prevented a small-size and space-saving design.
In the case where an electrically-conductive resin is used as the shield layer 104, anisotropy is encountered when the drain wire 103 is provided parallel to the conductor 101 in the conventional manner. The result is that a uniform shielding effect can not be obtained.
There is also disclosed a shield cable having no drain wire and utilizing an electrically-conductive resin. However, since high electrical conductivity can not be obtained, a practical use of it is difficult. Therefore, a metal braid or a metal foil is in practical use. However, the metal braid need to have a high braid density, and therefore tends to be heavy and expensive. The metal foil lacks in flexibility, and becomes deteriorated due to corrosion, thus failing to provide sufficient durability. Thus, these problems have been encountered.
Also, there are commercially available shield cables in which a metal foil, a metal braid or an electrically-conductive resin is provided, as an electrically-conductive layer, around a conductor insulator or a bundle of wires (Japanese Patent Application Unexamined Publication No. Sho. 64-38909). However, each of all the wires is formed into a shield wire, the wiring bundle has much space loss because of the circular cross-section of the wire. Thus, it is not suited for the space-saving purpose. Further, for connecting the electrically-conductive layer to the earth, a manual operation is required for separating the electrically-conductive layer from the internal conductor, and therefore the wiring can not be automated. Further, in the type of shield cable in which an electrically-conductive layer is provided around a bundle of several wires, induction between the wires within the bundle can not be prevented. When a metal foil or a metal braid is used as a shield layer, the construction is complicated, and therefore the efficiency of production of the cable is low, and a high cost is involved.
On the other hand, recently, in order to achieve the space-saving of the wiring, tape-like cables have been increasingly used, and there have been marketed a shield cable in which such a tape cable is enclosed by a metal foil or a metal braid as described above. Even with this wire, induction within the tape cable can not be prevented (Japanese Patent Application Unexamined Publication No. Sho. 61-133510/86).
Further, in the two, the type which uses metal as the shield electrically-conductive layer has a problem that it is heavy and inferior in durability.